1. Field of the Invention
This invention relates generally to a decoder for four channel FM stereophonic composite signals, and more particularly to a decoder for four channel FM stereophonic composite signals in which a signal indicative of the four channel stereophonic signal broadcasting mode is detected.
2. Description of the Prior Art
In U.S. Pat. No. 3,708,623 there is disclosed a four channel FM stereophonic transmitting system which is compatible with two channel stereophonic signal broadcasting and monaural broadcasting. With this system, it is necessary to switch the receiving condition of the receiver so as to adapt the same for four channel stereophonic broadcasting or two channel stereophonic broadcast reception. In general, a signal which is broadcast only in the four channel stereophonic broadcasting mode is detected for switching the receiver to the receiving condition suitable for four channel or two channel stereophonic broadcast reception.
Further, in U.S. Pat. No. 3,787,629, relating to "Apparatus for Distinguishing Between FM Broadcast Multiplex Transmission", it is proposed to distinguish between four channel and two channel stereophonic broadcast transmissions by detecting, in the four channel stereophonic broadcast transmission, a second sub-channel signal modulated on a second subcarrier which has a frequency twice that of a pilot signal and a phase difference of 90.degree. in respect to the latter, and which is not present in two channel stereophonic broadcast transmissions. Alternatively, this patent suggests that the four channel stereophonic broadcast transmissions may be distinguished by detecting therein a third sub-channel signal modulated on a third subcarrier having a frequency four times that of the pilot signal. However, since the level of the above mentioned second sub-channel signal or third sub-channel signal will vary, the detecting level will similarly fluctuate so that the detection or distinguishing of the four channel stereophonic broadcast transmissions from the two channel stereophonic broadcast transmissions cannot be reliably and stably achieved. Further, if the second or third subcarrier is of the suppressed carrier type, so that, as has been proposed, the four channel stereophonic broadcasting mode can be detected by suitably selecting the suppressed level, complicated detecting circuits are required for that detecting function.
It has also been proposed to provide a second pilot signal with a frequency three times that of a 19KHz first pilot signal in a frequency gap between the first subchannel and the third sub-channel. In this case, the four channel composite signal is applied to a phase locked loop (PLL) or a band pass filter to detect whether or not the second pilot signal exists and to determine whether or not a four channel stereophonic broadcast transmission is present on the basis of the detected signal. Further, in this latter proposal, the demodulated signal is applied to a second PLL or band pass filter to pick up the first pilot signal and discrete left front, left back, right front and right back audio signals are demodulated from the four channel stereophonic composite signal upon the reception of the latter. However, since a PLL or band pass filter is required to produce the first pilot signal and a separate or additional PLL or band pass filter is required to produce the second pilot signal, this last described proposal involves the use of an undesirably complicated circuit arrangement. Further, if the second pilot signal is produced by a band pass filter rather than a PLL with a view to relatively reducing the cost, such band pass filter must have rather sharp characteristics and, after assembly of the circuit, the same must be adjusted and care must be exercised to accommodate changes in its time constant and temperature.